The invention relates to a differential line driver circuit with a fully differential operational amplifier for driving a line signal output via a signal line.
This application claims the priority date of German patent application DE 100 45 720.7, filed on Sep. 15, 2000, the contents of which are herein incorporated by reference.
FIG. 1 shows a differential line driver circuit having a fully differential operational amplifier according to the prior art. The line driver circuit contains a fully differential operational amplifier OP having an inverting input (xe2x88x92), a non-inverting input (+), and a common mode signal input. The operational amplifier OP also has a first signal output (+) and a second signal output (xe2x88x92) that is in phase opposition therewith. The operational amplifier OP is supplied with voltage via a first supply voltage terminal having a positive supply voltage VDD, and via a second supply voltage terminal having a negative supply voltage VSS. The signal outputs of the operational amplifier OP are fed back to the signal inputs via feedback impedances RRI, RRII. The signal outputs of the operational amplifier OP are connected via matching impedances RAI, RAII to output terminals A1, A2 of the line driver circuit. The signal inputs are connected via gain adjusting impedances RE1, RE2 to input terminals E1, E2 of the line driver circuit. An input signal to be amplified, which is amplified by the operational amplifier OP and is output via the output terminals A1, A2 to the signal line with the line resistance ZL, is applied to the input terminals E1, E2.
The matching impedances RAI, RAII, are matched to the impedance ZL of the signal line in order to avoid signal reflections on the line.
It holds for the output impedance of the conventional line driver circuit shown in FIG. 1 that
Zaus=RAI+RAII=ZLxe2x80x83xe2x80x83(1)
Together with the impedance of the line, the matching impedances RAI, RAII form a voltage divider such that the output voltage Uopaus output by the differential operational amplifier OP must be twice as high as the output voltage Uaus to be made available at the output terminals A1, A2 of the line driver. It holds that
Uopaus=2xc2x7Uausxe2x80x83xe2x80x83(2)
It holds for the supply voltage Uversorgung of the operational amplifier that:
Uversorgung=VDDxe2x88x92VSSxe2x80x83xe2x80x83(3)
The supply voltage Uversorgung of the operational amplifier OP must always be higher than the output voltage Uopaus output by the operational amplifier:
Uversorgung greater than Uopaus=2xc2x7Uausxe2x80x83xe2x80x83(4)
In the conventional driver circuit, such as that shown in FIG. 1, the supply voltage Uversorgung must be at least twice as high as the output voltage Uaus of the driver circuit. The necessary supply voltage Uversorgung is therefore relatively high. Because of the relatively high supply voltage, the conventional driver circuit illustrated in FIG. 1 consumes significant power.
It is therefore an object of the present invention to create a differential line driver circuit having a fully differential operational amplifier that requires a low supply voltage and has a low power loss.
The invention creates a differential line driver circuit having a fully differential operational amplifier for the purpose of driving a line signal output via a signal line, having
a first input terminal and a second input terminal for the application of an input signal,
a fully differential operational amplifier that has an inverting signal input connected to the first input terminal, a non-inverting signal input connected to the second input terminal, a first signal output for outputting an amplified output signal in phase with the first input signal, and a second signal output for outputting an amplified output signal in phase opposition with the input signal.
Gain adjusting impedances provided in each case between an input terminal of the line driver circuit and a signal input of the operational amplifier,
a first feedback impedance connected between the inverting signal input of the operational amplifier and the first signal output of the operational amplifier,
a second feedback impedance connected between the non-inverting signal input of the operational amplifier and the second signal output of the operational amplifier,
a first matching impedance connected between the first signal output of the operational amplifier and a first output terminal of the line driver circuit,
a second output impedance, is connected between the second signal output of the operational amplifier and a second output terminal of the line driver circuit,
the differential line driver circuit according to the invention additionally having a first positive feedback impedance connected between the first output terminal of the line driver circuit and the non-inverting input of the operational amplifier, and a second positive feedback impedance connected between the second output terminal of the line driver circuit and the inverting input of the operational amplifier, and
the output impedance of the line driver circuit being matched to the impedance of the line.
The output impedance of the line driver circuit is preferably determined by the product of an output impedance synthesis factor and the sum of the impedances of the two matching impedances.
The output impedance synthesis factor can preferably be adjusted as a function of the positive feedback impedances and the feedback impedances.
In one embodiment of the differential line driver circuit according to the invention, the first positive feedback impedance and the second positive feedback impedance have the same impedance.
The first feedback impedance and the second feedback impedance likewise preferably have the same impedance.
The output impedance synthesis factor is preferably greater than one.
In a particularly preferred embodiment of the differential line driver circuit according to the invention, the output impedance synthesis factor is approximately five.
The differential line driver circuit according to the invention is preferably of fully symmetrical design.
The signal driven by the differential line driver circuit is preferably an xDSL signal.
In a particularly preferred embodiment, the adjusting impedances, the feedback impedances and the positive feedback impedances as well as the matching impedances are complex impedances.
The impedance values of the adjusting impedances, the feedback impedances, the positive feedback impedances and the matching impedances can preferably be switched over by switching devices.
The fully differential operational amplifier preferably has a common mode signal input.
These and other features and advantages of the invention will be apparent from the following detailed description and the accompanying drawings in which: